Phase inversion circuits



S. HUNT PHASE INVERSION CIRCUITS Sept. 10, 1940.

Filed Feb. 12, 1938 I Pro PUSH-PULL AyPL II II L www- 24 gig;

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Patented Sept. 10, 1940 PATENT QFFlCE PHASE INVERSION CIRCUITS Seymour Hunt, Jackson Heights, Long Island, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 12, 1938, Serial No. 190,206

4 Claims.

My present invention relates to phase inversion networks for audio push-pull amplifiers, and more particularly to tube circuits capable of multiple functions including at least audio phase inversion for push-pull amplifiers.

In the past there have been suggested various arrangements for coupling an audio voltage source to a push-pull audio amplifier by means of a phase inversion network adapted to develop from the source a pair of audio voltages in phase opposition. However, such inversion networks have usually not been capable of performing additional functions. For example, it is of advantage to secure audio amplification in the inversion network.

It may be stated, therefore, that it is one of the main objects of my invention to provide an audio phase inversion tube adapted to provide amplification of the audio voltage impressed thereon; the tube being additionally constructed and arranged to provide a pair of audio voltages in phase opposing relation.

Another important object of my invention may be stated to reside in the provision of an electron discharge tube provided with amplifier and phase inversion sections; an audio voltage source being coupled to the input electrodes of the amplifier section; and an audio connection being arranged between an input electrode of the inversion section and the output electrode of the amplifier section so that a pair of audio voltages may be derived from the inversion section in phase opposed relation.

Another object of the invention is to provide a single tube with a diode section which is adapted for detection of signal energy; and a phase inversion section which is adapted to utilize detected energy and produce therefrom a pair of audio voltages in phase opposed relation.

Still other objects of my invention are to improve generally the efficiency and operation of audio phase inversion networks, and more especially to provide audio phase inversion tubes capable of audio amplification, and which are economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

Referring now to the accompanying drawing.

wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 an audio circuit capable of securing audio phase inversion for push-pull amplifier driving, and at the same time amplifying the audio voltage prior to the inversion step. In this circuit it is to be understood that the numeral i denotes an audio transformer whose primary may be coupled to any desired source of audio energy. For example, the source of audio frequency energy may be the detector of a radio receiver; a phonograph pick-up device; or the microphone of a public address system. Regardless of the nature of the audio voltage, it is desired to provide a network which will feed the audio voltage developed across the secondary of transformer l to the control grids of a pair of audio amplifier tubes arranged in push-pull relation, for example a class A type of amplifier circuit. In order to accomplish this it is essential to provide a circuit which will be capable of deriving from the audio voltage at l, a pair of audio voltages which are 180 out-of-phase. It is also desirable to derive these phase opposed voltages in amplified form, and, of course, this should be accomplished with a minimum of circuit elements.

Accordingly, the circuit in Fig. 1 employs a tube 2 which may be of the so-called pentagrid type. Such a tube is commercially designated as a 6A? type tube, and comprises a cathode 3, a plate 5 and five intervening electrodes. The electrode 5, adjacent the cathode 3, is connnected to the high potential end of the secondary Winding of transformer l; the usual resistor-shunt condenser network ii being provided in the grounded cathode lead to produce the normal negative bias for the signal control grid 5. The second grid 1 is connected to a source of positive potential (+B) through a resistor 8, and the electrode end of resistor 8 is connected by lead 9 and condenser ill to the fourth electrode II. The leak resistor l2 connects the electrode side of condenser It to ground; while the grids l3 and M are disposed on opposite sides of electrode H in order to provide a positive field around eleca receiving system embodying a trode II. The electrodes I3 and I4 are connected in common to a source of positive potential (-l-B) through load resistor I5. The plate 4 is connected to a source of positive potential (+B) through a load resistor I6. It will be understood that the control grid of one of the push-pull amplifiers is coupled to the anode end of resistor I6 through coupling condenser I1, while the control grid of the opposite push-pull tube is connected to the electrode side of resistor I5 by coupling condenser I8. The push-pull arranged tubes are not shown since those skilled in the art are fully aware of the construction thereof.

The audio voltage applied to grid 5 is amplified in the amplifier section including cathode 3, grid 5 and output electrode '1. The amplified audio Voltage is developed across resistor 8; and, then, is applied to grid II by coupling condenser III. The audio voltages developed across each of load resistors I5 and I6 are 180 degrees out-of-phase. Hence, they are suitable for impression on a pushpull audio amplifier network. Further examination of the action of the tube 2 reveals that cathode 3, grid 5 and electrode 1 provide a virtual cathode. There is thus secured audio amplification in the triode section of the tube; audio amplification and phase inversion are secured in the section I 4-III34. Instead of the side rods used for electrode I in a GA? type tube, a wound grid may be employed. It will be understood that the out-of-phase relation is secured for the voltages across resistors I6 and I5, because of the negative mutual conductance to electrodes I3 and It. That is, when the grid 5 is swung negative the current through resistor I 6 decreases, while the current through resistor I5 increases. Hum, due to heater cathode leakage, is not a factor in this arrangement, since the cathode 3 connects to ground.

To show the improved audio gain secured by this circuit arrangement, let it be assumed that the section 35-7, the triode amplifier, has an amplification factor of 5. If 1 volt were applied to the grid 5, then the arrangement acts as though 5 volts were introduced into the virtual cathode circuit of the section I4I I I 34. When input voltage is applied to the cathode of an audio amplifier, the plate voltage developed is approximately the gain of the tube plus 1 if the load resistor is of correct value. If a gain of 3, for example, is secured in the section I l-I II34 by the application of 1 volt to grid II, then by applying 1 volt to the grid 5 the overall gain of the two sections would be about 20. Hence, it will be seen that phase inversion has been secured with considerable gain of audio voltage. The load resistors I5 and I6 need not be necessarily equal; since if the plate and screen currents were unequal, then the resistors could be made unequal to an extent such that the voltages thereacross were equal.

The arrangement in Fig. 2 differs from that shown in Fig. 1 in the utilization of a tube having a pair of independent screen grid amplifier sections. The tube is denoted by the numeral I9. The audio signal is impressed on grid 20 of the lower amplifier section; the audio voltage developed across load resistor 2| is impressed by coupling condenser 22 on the input grid 23 of the upper section. The latter comprises cathode, or emission, surface 24; screen grid 25; input grid 23, and plate 26. The audio voltages for the push pull stage are derived from plate and screen grid resistors I6 and I5 respectively; the audio coupling condensers I! and I8 impress the phaseopposed voltages on the push-pull connected audio tubes. The emission surface 30 is at the same direct current potential as the surface 24; the bias resistor 6 provides the operating negative bias for grids 23 and 20, and the grounded grid leak resistor I2 is employed as in Fig. 1. A triode section can be used in place of the lower screen grid section, if desired.

In the arrangement of Fig. 3 there is shown a method of securing diode detection and phase inversion in a tube of the 6A? type; the same type of tube may be used as at 2 in Fig. 1. The tube 3! in Fig. 3 has its cathode 32 grounded; the second grid 33 acts as the anode of a diode detector, and is connected to thegrounded cathode through the resonant input circuit 34 and load resistor 35 arranged in series. The circuit may be tuned to an intermediate frequency if the receiver is of the superheterodyne type; the usual amplifier and first detector networks til will feed circuit 34. The carrier by-pass condenser 36 is shunted across load resistor 35; an automatic volume control connection, designated AVC, applies the direct current voltage developed across resistor 35 to one or more of the tubes in network 48., across resistor 35 is applied to grid H (the fourth grid of the tube) by the potentiometer t24" 44; the resistor 33 is connected to a negative potential point, so that grid lI may be properly biased for audio amplification. Adjustment of tap 42 varies the audio input to the audio phase changer. The audio voltages across plate resistor 50 and screen resistor 5I are in phase opposition for the reasons stated in connection with The audio voltage developed Fig. 1; these voltages are applied to the grids of the following push-pull amplifier.

In Fig. 4 is shown a modification of the invention, wherein tube 60 has the audio input signal applied to grid iii. The second grid 62 is connected to a source of positive potential through load resistor I0 having a magnitude of approximately 50,000 ohms. The fourth grid is 62 and is at a negative potential with respect to cathode; the third and fifth grids are connected to a source of positive voltage through a half megohm resistor. The plate of the tube is connected to a positive voltage source through a 50,000 ohm resistor I2. The voltages across load resistors I0 and I2 are in phase opposition; they are applied to the push-pull connected tubes. When grid BI is swung positive by signal voltage, then grid 62 assumes an instantaneous negative potential. Concurrently, the plate end of resistor I2 assumes a positive potential with respect for audio. A two-voltaudio input, with this arrangement, produces approximately 17 volts across each of resistors 10 and I2. Grid 62 is a wound grid; and grid 62 may be left open.

systems for carrying my invention into efiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a tube having a cathode and a plurality of cold electrodes arranged in succession in the electron stream from the cathode, a source of signal voltage coupled between a first of said cold electrodes nearest said cathode and to the B supply which is at ground potential :60 While I have indicated and described several a point or relatively fixed alternating current potential, means connected between said cathode and said point establishing said first electrode at a negative direct current potential, a load resistor connected to, and individual to, each of a second and third cold electrode of said tube, means establishing said second and third electrodes at positive potentials, means connecting an additional cold electrode of the tube to said point thereby to apply a negative potential thereto relative to said cathode, said additional electrode being located between the second and third cold electrodes, and means for deriving from said load resistors amplified signal voltages in phase opposition.

2. In combination, a tube having a cathode, signal grid and two output electrodes, the grid being disposed between the two output electrodes, a load resistor connected to each output electrode whereby audio voltages developed across the load resistors are in phase opposition, additional electrodes in said tube providing an audio amplifier, a source of audio waves coupled to the amplifier input electrodes, and means coupling the amplifier output electrodes to said signal grid, said additional electrodes being disposed in the electron stream between the cathode and one of said output electrodes.

3. In combination with a tube having a cathode, a plate and at least four grids arranged in succession therebetween, a source of audio waves coupled to the cathode and the first of said grids, a load resistor connected to the second grid for developing an amplified audio voltage thereacross, an audio coupling means impressing said amplified voltage upon the fourth grid, a load resistor individual to each of said plate and third grid, and the audio voltages across the individual plate and third grid resistors being in phase opposition.

4. In combination with a tube having a diode section and a phase inversion section, a radio input circuit coupled to the diode section and including means for deriving audio and direct current voltages from radio waves, means impressing the audio voltage on an input electrode of the inversion section, the latter including a pair of output electrodes arranged in such a manner with respect to the said input electrode that audio output voltages in opposed phase are developed, said two sections having a common cathode, one of the output electrodes being a plate and the remaining electrodes of both sections 26 being disposed between the cathode and plate.

SEYMOUR HUNT. 

